1. Field of the Invention
This invention relates to a solid state laser which can have large power output and generate a high-quality laser beam.
2. Description of The Related Art
Referring first to FIG. 2, there is shown the configuration of a conventional solid state laser disclosed in the U.S. Pat. No. 4,918,704 (1990). In this figure, reference numeral 1 represents a rod type solid-state laser medium (hereunder sometimes referred to simply as a laser rod); 2 a total reflection mirror; 3 a partially transparent output coupling mirror (hereunder sometimes referred to simply as an output mirror), the reflectivity of which changes according to a Gaussian-like function in the radial direction (namely, in the direction perpendicular to an optical axis 9 of a laser resonant cavity); 4 a flash lamp for illuminating and pumping the laser rod 1; and 5, 6 and 7 solid state elements for Q-switching. Namely, reference numeral 5 denotes a dielectric multi-layer polarizer; 6 a quarter wave plate; and 7 a Pockels cell.
The radius of curvature of a reflection surface 2A of the mirror 2 is established in such a manner that the rays of laser light reflected by the mirror 2 become parallel rays. Further, the radius of curvature of a reflection surface 3A of the mirror 3 is established radius of curvature of the reflection surface 2A of the mirror 2, namely, is established in such a fashion that laser light progresses in the cavity to the mirror 2 expanding a beam diameter or radius and is then incident on the reflection surface 2A of the mirror 2 so as to cause laser oscillation. Incidentally, an output surface 3B of the mirror 3 has the same radius of curvature as the reflection surface 3A. Furthermore, the laser rod 1 has a property of giving rise to a thermal lensing effect when undergoing an optical pumping.
With the above described configuration, the conventional solid state laser operates as follows. First, the laser rod 1 is pumped by the flash lamp 4. Thereafter, the Pockels cell 7 is activated when the pumping degree or rate of the laser rod 1 reaches a maximum value. Then, an optical path provided in the resonant cavity, which has been intercepted by the polarizer 5 and the quarter wave plate 6, becomes transparent optically. Thus the laser rod 1 comes to perform a lasing operation. Consequently, a laser oscillation is commenced in the resonant cavity.
Subsequently, as illustrated in FIG. 3, laser light reflected by the reflection surface 3A of the mirror 3 is amplified while going through the laser rod 1, expanding the beam diameter. The laser light is then incident on the total reflection mirror 2, further expanding the beam diameter. Then, rays of the laser light are reflected by the reflection surface 2A of the mirror 2 and become parallel to the optical axis 9 of the resonant cavity. Afterwards, the laser light is incident on the laser rod 1 again and then is amplified while going through the laser rod 1. Finally, the laser light is incident on the reflection surface 3A of the output mirror 3 parallel with the optical axis 9 of the resonant cavity. As described above, the reflectivity of the reflection surface 3A of the mirror 3 is represented by a Gaussian-type function. Thus a transverse mode of laser output light, which is transmitted by the mirror 3 and is then output to the outside of the laser oscillator, has a smooth profile and becomes parallel to the optical axis 9 of the resonant cavity.
Meanwhile, when the temperature of the laser rod 1 becomes high to the extent sufficient to cause a thermal lensing effect, the course of laser light travelling through the inside of the laser rod 1 is changed or turned. Thus, when the thermal lensing effect is caused, the laser light travelling through the resonant cavity is incident on the output mirror 3 by being converged or diverged. As a result, laser output light drawn out of the resonant cavity converges or diverges.
Thus, if input power as radiated pumping light is increased in order to also increase output power of the laser oscillator, thermal lensing effects of the laser rod 1 become increased. As a consequence, laser light is incident on the mirror 3, converging or diverging. Then, the laser light is output from the output surface 3B of the mirror 3, converging and diverging.
Meanwhile, the wavelength converting efficiency of a non-linear crystal (not shown) decreases when a diverging angle of incident laser light increases. The conventional solid state laser, accordingly, has a drawback in that when the laser output light is made to be incident on the non-linear crystal, the wavelength converting efficiency falls and high-average-output laser light, the wavelength of which is converted, cannot be obtained.
Further, to eliminate this drawback, it is devised to enlarge the radius of curvature of the reflection surface 2A of the total reflection mirror 2 in such a way that the rays of laser light reflected by the mirror 2 becomes parallel rays when the laser light is emitted from the laser rod 1.
However, if the radius of curvature of the surface 2A of the mirror 2 is increased, the laser light reflected by the reflection surface 2A and transmitted by the laser rod 1 becomes as illustrated in FIG. 4. Therefore, the conventional solid state laser has another drawback in that useless regions 10, which do not contribute to a lasing operation at the time of performing laser oscillation, increase in the laser rod 1.
The present invention is created to eliminate the drawbacks of the conventional solid state laser.
It is, therefore, an object of the present invention to provide a solid state laser which can efficiently obtain parallel rays of high-quality laser output light having high-average-output power and a converted wavelength even when a thermal lensing effect of a solid-state laser medium becomes enlarged by increasing input optical-pumping power.